The present invention relates to a method and an apparatus for fuel combustion with low emission of NO.sub.x, soot and particulates, and is particularly suited to combustion of very heavy products with relatively high pollution potential.
There is currently a growing trend to convert hydrocarbonaceous fluids to valuable products and to reduce the quantity of less valuable products. Refineries are being equipped with advanced conversion units, which are designed to increase the amount of distillate from a given feedstock. In consequence of this practice, the bottom products generated become ever heavier, with higher residual carbon content and fuel nitrogen concentrations. Since these bottom products still have a certain quantity of thermal energy, it is worthwhile to use these products in combustion equipment in combination with steamboilers, furnaces, and the like.
Increase of the residual carbon content and fuel nitrogen concentration of the fuels to be fired may involve an important problem, in that such increases are normally associated with higher NO.sub.x, soot and particulates emission when applying currently available combustion equipment. Especially in highly industrialized areas, the emission of NOx, soot and particulates may be assumed to increase drastically in the forthcoming years, if special measures are not taken. This fact explains the growing need for preventing pollution of the atmosphere due to excessive emission of the above unhealthy substances.
There are, in principle, two solutions possible for dealing with this problem. The first solution is the cleaning of the flue gases prior to emission into the atmosphere. This solution is, however, very expensive since very special cleaning equipment and processes are necessary, and the cleaning processes themselves reduce the efficiency of the total installation. The second option for reducing emission of NO.sub.x, soot and particulates is to improve the combustion processes and equipment in such a manner that the generation of these pollutants is minimized or considerably reduced. In order to reduce soot and particulates emission, the mixing intensity of the fuel and the combustion air may be enlarged. In this way, successful attempts have been made in the past for reducing soot and particulate emissions from combustion units. Furthermore, methods have already been developed for reducing NO.sub.x emissions. It has, however, been found that attempts to reduce NO.sub.x emissions are, in general, associated with an increase in soot and particulates emissions. In this context, it is noted that, in the past, burners for low NO.sub.x emission have been proposed which are able to operate at a very low combustion air velocity and able to atomize the fuel sufficiently. A proper atomization of heavy fuel can only be attained with a high atomizing steam consumption. For this type of low air velocity burner, the particulate emissions can be kept rather constant when reducing the NO.sub.x emissions. Such a burner will, however, be sensitive to fouling when fired in a vertical position due to the applied high atomization of the fuel. The low combustion air velocity which should prevail suggests the use of a burner with a relatively large diameter, which will produce non-uniform heat flux distributions. A further disadvantage of this type of burner is imposed by the fact that the required high atomizing steam consumption reduces the fuel economy considerably.
Since there will be a growing supply of fuel with an increased residual carbon content in the future, the available combustion methods will most probably become insufficient for meeting the environmental requirements without substantially reducing combustion efficiency. It is, therefore, an object of the invention to provide a fuel combustion method suitable for heavy fuels in which method the emissions of NO.sub.x, soot and particulates are minimized or considerably reduced when compared with known combustion methods, without adversely affecting the fuel economy.